In RF communications systems, such as telephone or cable-television systems (CATV), signal transmission conductors are frequently exposed to transient overvoltages. These transients may be caused, for example, by lightning, the switching of nearby power cables, or field operations such as “hot-coring,” in which live transmission cables are cut for connector attachment, in turn generating transients of up to 150 V. If these transients are not prevented from reaching sensitive components in the communication system, such as semiconductor line amplifiers, the components may be severely damaged.
Circuits for limiting the electrical energy reaching a device through a conductor have existed for many years. Protection circuits may include silicon diodes, PIN diodes, Schottky diodes, shunt bipolar transistors, insulated gate field effect transistors (IGFETs), metal oxide field effect transistors (MOSFETs), and metal-semiconductor field effect transistors (MESFETs).
FIG. 1A shows, for example, a conventional MOSFET-based circuit described in U.S. Pat. No. 5,543,649 to Kim et al. In this circuit, the gate 1 and the P+drain 2 of a P-channel dielectric gate MOSFET are coupled to an input voltage Vx, and the P+ source 3 of the MOSFET is coupled to a substrate voltage Vss. As can be seen from FIG. 1B, a PNP-type parasitic transistor composed of the P+ source and drain regions 2 and 3 and the N-type substrate 4 is also formed. The potential of the substrate voltage Vss is placed between a negative voltage and zero volts, and the N-type substrate 4 is coupled to the substrate voltage Vss or to some other reference voltage. This protection device utilizes a junction breakdown which occurs between the P+ drain and the N-type substrate when the protected input (drain) voltage is 0.7 V greater than the source voltage.
However, a MOSFET-based circuit such as the one shown in FIG. 1A is unsuitable for application to line amplifiers included on a gallium arsenide (GaAs) monolithic microwave integrated circuit (MMIC). First, it requires the use of both N- and P-channel devices. Second, it is designed for use on an unbalanced signal line. Since line amplifiers are frequently applied in a balanced-signal configuration in combination with an off-chip balun transformer, two circuits of the type shown in FIG. 1A would have to be used to protect the two phase legs of the balanced signal—or, alternatively, an off-chip MOSFET device would have to be connected after the balun transformer—thus adding to the cost of the protected circuit. Third, although overvoltage transients are frequently dampened sinusoids having both positive and negative voltage swings, the circuit of FIG. 1A cannot clamp the negative portions of an overvoltage transient except by breakdown of the MOSFET. Fourth, it can only be used where the input voltages are small, because the parasitic PNP transistor turns on whenever the source voltage is 0.7 V less than the input (drain) voltage. Finally, it adds a large amount of harmonic and intermodulation distortion to an upstream line amplifier. In order to prevent the protection circuit from adding to harmonic or intermodulation distortion in highly linear amplifiers, protection transistors must be deeply biased below threshold to ensure that very high impedances are presented.